why is it so small .. would be proc je tak male ? and what about why is that so small?
Yes, can be.
Would proc je tak maly be why is he so small?
yes
And the exercise sentence --"Proč je tak malá?" -- could also mean, "Why is it small," if we were talking about something that is grammatically feminine.
Why is "why is he so small" marked as a typo ( (s)he )?
Because only "she" is correct - "malá" is feminine.
"Why is he so small?" is just wrong here, Duo mistakenly considers that a typo.
It could also be "Why is it so small?" if you're talking about an item that is feminine in gender.